Know Your Precast Refractories – and the Challenges They Bring

Did you know that precast refractory shapes are a critical part of industrial production in demanding, heat-intensive industries? These products are subject to temperature variations, mechanical stresses, chemical attack, mechanical erosion, and thermal cycling issues, to name a few of the challenges. Finding a way to solve your toughest performance issues may lead to reduced lead times, lower operating costs, and longer-lasting, cost-effective shapes.

What is precast?

Precast refractory shapes are high-temperature ceramic shapes that are designed to hold or contain high temperatures – i.e., molten metal in foundries, steel forging, or other high-heat processes. Precast shapes are a vital consumable product needed in the making of any cast or formed metal product.

Creating a precast shape that can withstand heavy use and perform under stress requires attention to detail and product knowledge. To design and manufacture a precast product that meets your specific production needs, it’s critical to start with an understanding of the production environment and the causes of potential failure.

All the environmental conditions endured by precast shapes (thermal cycling, chemical attack, mechanical abuse) contribute to its success or failure. These shapes are a specific combination of dry material combined with a specific water content requiring predetermined mixing times, timed vibration of the shape, as well as a specific length of time before it can be stripped out of its mold. Creating a precast that will endure heavy use and tolerate daily stress is a five-stage process that requires attention to detail and proficiency at every step: design, form, cast, fire, and ship.

Manufacturing precast shapes entails a number of mixing and casting process steps, including: applicable base material and material additives, specific water content, mixing method (mixer type) and mixing time, vibrating time and frequency, selecting the right mold-release agent, and setting proper curing conditions – all of which vary depending on the chemistry of the material and the size and shape of the mold.

Each step must be executed perfectly in order to ensure acceptable surface quality, minimal air pockets, and the resulting clean edges with minimal flashing.

Finally, after air curing, the shape will be furnace fired at a specific time and temperature program per the engineering standards of the specific base product.

Ensuring each of the five processes addresses all the specifications required for the final application –steel, iron, ceramic or other industry use – is the recipe for success.

Design … Planning the precast shape

Precast-shape manufacturing starts with a mold or pattern to form the shape - which means a design. Existing prints of the shape (or the actual shape itself) are always a plus for the next build. However, field measurements or a site visit may be required for details. Mold-making and the materials used to build the molds routinely employ different options based upon the type of mold required – which will vary depending on size, pieces required, complexity, and the dimensional tolerances required in the shape.

For example, the quantity of shapes required may determine which material may be used for maximum life. For simplistic low-volume shapes, plywood or Styrofoam forms do the job. Shapes with very tight tolerances may necessitate the use of a more complex mold made from composite materials, plastic, or metal.

Prior to selecting a material, the environment and failure causes must be understood along with how those will impact the precast shape. Different environments require different precast materials. Copper and steel, for example, require different qualities from precast refractory materials.

Form … Manufacturing the mold

Creating the mold that will form the precast shape is the second step in the process. In addition to traditional mold capabilities, firms (like Onex) that offer rapid mold prototyping allow for short-run trials to prove new products, or to increase production to meet additional product needs.

There are benefits to each type of mold. Permanent molds such as steel are a more expensive option, but longer-lasting, whereas temporary molds like wood and Styrofoam are typically the quickest and cheapest to make, the latter being extremely easy and virtually free to accommodate post-production design changes. All the cost implications and benefits should be considered without prejudice of material type.

Molds are initially prepared to take the cast material. Prior to pouring and even mixing the material, the molds are prepped with a release agent. Every precast shape that is produced requires a specific release agent, which may vary based on casting heat and materials, so that it can be extracted from the mold intact.

Release agents are critical to successful demolding, as well as surface quality and final appearance. With the correct release agent - whether petroleum-based lubricants or other additives for ease of release - you can reduce sharp edges, air pockets, and flashing while improving overall surface quality.

Cast … Mixing and pouring

Once the mold is designed, the refractory materials selected for the shapes need to be mixed, vibrated, and poured into the prepared mold. Some shapes may benefit from different types of refractory and some may need certain additives to increase strength and improve performance, so they can hold up during any process. As a manufacturer, what you are looking for is the best product option for the application, rather than being locked into a single refractory option.

The different materials required (application-specific) react differently to various mixing actions, for proper material dispersion. Equipment may include: pan mixer, rotary mixer, and Hobart mixer – because materials react differently to the various mixing “actions” and different mixer styles.

Once the material has been poured into the mold, vibration is applied to remove any air pockets that formed during mixing. Additionally, special care should be taken to allow for proper air curing in a strictly controlled environment for temperature and humidity.

Fire … Cure and cool down

The firing of a precast shape, and the final cooling of the shape, are equally important. Unintentionally fast-fired shapes may thermally spall/crack, as might fired pieces that are not properly cooled, leaving unusable materials instead of a reliable precast product. With a controlled furnace cool-down process even pieces as thick as 18 inches can be successfully precast.

The watch points for the furnace output on a precast part are steady temperature maintenance, proper circulation, and controlled cool-downs, so that parts are not subject to temperature variations. Working with a supplier that has refractory furnace expertise not only ensures that customers source precast with the right specifications, but it gives them more options. With furnace capabilities over 2000° F, high firing is an option to create a ceramic bond.

Quality control and shipping

Quality control is the last stop before shipping. After the precast is molded, formed, and tested it goes to quality control, wrapping, palletizing, and then it is loaded on the truck.

During the packaging process, the shipping materials will be custom to the precast shape, to prevent damage. The pallet itself can be specialized to the parts needs: specific pallet size, additional wood thickness for strength, complete crating for protection, Styrofoam for part protection, etc. Additionally, shrink wrap options could include choices in thickness and color. Customization for shipping options allows for successful, intact delivery.

Next time you look at replacing or updating your precast parts to improve your uptime and costs, review your supplier’s stance on the five steps outlined above. These steps are critical to ensuring that you receive the exact precast product with the properties you need - or even better.

Michael Ducato is the Vice President of Business Development at Onex, a U.S. designer and manufacturer of precast refractories.